This invention relates to a decanter centrifuge. In a specific application, this invention relates to a decanter centrifuge with means for controlling the moisture content of a discharged cake or solids fraction. This invention also relates to an associated method for operating a decanter centrifuge.
A decanter centrifuge generally includes an outer bowl, an inner hub carrying a worm conveyor, a feed arrangement for slurry to be processed, and discharge ports for cake solids and clarified liquid. The bowl includes a cylindrical section and a conical beach section. The bowl and the hub are rotated at high, slightly different angular speeds so that heavier solid particles of a slurry introduced into the bowl are forced by centrifugation into a layer along the inner surface thereof. By differential rotation of the worm conveyor and the bowl, the sediment is pushed or scrolled to a cake discharge opening at the smaller, conical end of the bowl. Additional discharge openings are provided in the bowl, usually at an end opposite of the conical section for discharging a liquid phase separated from the solid particles in the centrifuge apparatus.
One of the goals in centrifuge operation is to produce cakes with a low moisture content. Among factors contributing to a low cake moisture content are a long residence time and a high compacting pressure. The compacting pressure is related to the G level (centrifugal acceleration) and the cake height. The compacting pressure generated by a column of sludge varies with the radial distance from the bowl wall. It is highest at the bowl wall and decreases radially inward. FIG. 10A shows a typical result of raw mixed sewage sludge which is compacted in a laboratory spin tube (1.3 inches in diameter and at a radius of 8 inches) under a force determined by the G level and the cake height. In one test, the cake height is about 2 inches and is compacted under 2000 g. In another test, the raw mixed sewage sludge is subjected to 900 g with a thicker cake pile of 2.6 inches. In both cases, the cake solids profile is stratified with the driest cake at the largest radius, adjacent to the outer wall of the spin tube.
In addition, it is known to form a dip weir along the outer surface of the conveyor hub, at or about the location of the junction between the cylindrical and conical sections of the bowl, to serve in selecting the driest portion of the cake at the discharge end of the bowl. The dip weir blocks the transport of the sludge cake in such a manner that the most compacted part of the cake passes under the dip weir and reaches the cake discharge opening. The dip weir also acts to provide the appropriate resistance to cake flow so as to maintain a large cake thickness upstream of the weir, creating high compacting pressure and long residence time. In conventional practice, the dip weir is fixed to the hub so that the radial gap between the outer edge of the dip weir and the inner surface of the bowl is constant or fixed. The designer must position and dimension the weir to minimize cake moisture content while not increasing cake transport resistance through the gap so as to unduly limit the solids capacity of the machine. The optimal gap height depends on the nature of the cake, the G level, and the cake flow rate or solids throughput. The designer is forced to guess at the correct gap height, guided somewhat by past experience.
Another application for a decanter centrifuge is in three-phase separation (as in oil, water and solids) wherein typically two lighter liquid phases (e.g., oil and water) are discharged at the large end of the decanter centrifuge and the heaviest solid phase, settled adjacent to the bowl wall, is discharged at the smaller conical end of the centrifuge. In a three-phase separation process, centrifugation stratifies the phases because of their density differences. A problem with three-phase separation is that the lightest phase (oil) is typically entrained by the solid phase as it emerges out of the oil-water pool in the conical section. The quantity of oil carried along by the cake solids depends on several factors including the surface velocity of the cake and the product of the centrifugal acceleration and the sine of the climb angle. The surface velocity of the cake is related to the differential speed of the conveyor and the bowl, the cake height or solids throughput, and the cake theological properties.